Office Communicator 2007 and R2 File Transfer when Forefront Security for Office Communications Server is installed.

Communicator 2007 or Communicator 2007 R2 File transfers no longer occur peer-to-peer after installation of Forefront Security for Office Communications Server 2007.

By default, IM-based file transfers occur as a peer-to-peer file copy between two instances of Office Communicator. When FSOCS is installed, it redirects the file transfer through the server it is deployed on so the file can be evaluated against filters and scanned for malware before it is sent to the intended recipient.

Firewall ports, both inbound and outbound, on the server where an instance of FSOCS is installed need to be opened for the TCP connections. The default port range includes ports 6891 through 6901.
More information on configuring the default file transfer port range can be found in the File Transfers topic in the Introduction to FSOCS section of the FSOCS user guide.